I've actually thought about this happening in A2 since in Ultimates 2 Widow betrays the team. It would work with Whedon talking about how he's going to dig a knife into the team.

Whedon said there will be 1-2 new team members, which will probably Falcon and/or Iron Patriot. That's all I'm seeing in terms of new members though.

Avengers 2 will be the "Empire Strikes Back" of the movies, which sounds great to me. I'm excited to see them dig deeper into the characters. Now we just need RDJ to re-sign, and everyone else is already on contract.